A polyvinyl chloride resin is a homopolymer of vinyl chloride or an interpolymer comprising 50% or more of vinyl chloride. The polyvinyl chloride resin is inexpensive and easy to control hardness and can be applied to various processing processes such as foam molding, extrusion molding, injection molding, calendering, etc. Also, since it provides molded articles with superior physical/chemical properties, it is extensively utilized in various fields.
However, because the polyvinyl chloride resin has disadvantages in terms of impact strength, workability, thermal stability and heat distortion temperature, additives to address these disadvantages are used. For example, additives for the polyvinyl chloride resin such as impact modifiers, processing aids, stabilizers and fillers are suitably selected according to applications thereof.
Recently, interest in foam molding of the polyvinyl chloride resin for miniaturization, reduction in weight, high performance and cost saving of molded articles has increased. However, when the polyvinyl chloride resin is foam-molded alone, there are disadvantages in that sufficient elongation and melt strength cannot be obtained and appearance of molded articles is thus bad. Moreover, foam cells are large and non-uniform and foaming magnitude is thus low.
In order to solve these disadvantages, it is suggested to use a processing aid in combination with a foaming agent in the polyvinyl chloride resin. The processing aid improves inherent melting delay characteristics of the polyvinyl chloride resin, thereby helping to fully exert all mechanical and chemical properties.
Specifically, when the polyvinyl chloride resin is foam-molded, the processing aid helps to sufficiently proceed melting of the vinyl chloride which forms walls of the foam cells so as to increase a high-temperature melt strength and to resist pressure of gas decomposing and expanding at high temperature, thereby preventing the open cell structure, in which the walls of the foam cells bust to be connected to each other, from being formed. The processing aid early breaks boundaries of primary particles, which are basic constituent units, by such an effect during foam molding to promote the uniform molten state on a molecular level, so that the product as the molding completed may result in obtaining a molded article with uniform mechanical and chemical properties.
The processing aid for the polyvinyl chloride resin which is currently commercialized comprises methyl methacrylate alone, which is excellent in compatibility with the polyvinyl chloride resin, as a main monomer. Or it comprises high molecular weight acrylic polymer prepared by copolymerizing methyl methacrylate with a comonomer such as acrylate, methacrylate, a monomer with nitrile unsaturated double bond or a monomer with aromatic unsaturated double bonds through emulsion polymerization.
For example, U.S. Pat. No. 6,221,966 discloses preparation of a processing aid by polymerization of a latex polymerized using alkyl acrylate as a main monomer and methyl methacrylate.
U.S. Pat. No. 6,391,976 discloses that a monomer of methyl methacrylate and butyl methacrylate or polyethyl methacrylate can be used to improve foam molding properties.
Although the acrylic processing aids proposed in these patents have improved foam moldability to a certain extent, their effects are not sufficient. Furthermore, when the acrylic processing aid was used, high melt viscosity increased to lower workability and dispersibility. In order to compensate for this, methods such as using styrene-acrylonitrile copolymer, controlling contents of the comonomer or using separate additives were suggested, but problems such as decrease in foaming efficiency due to change of material quality and increase in cost due to adding additives have rather occurred. Therefore, there is a need for more studies on an acrylic processing aid having excellent aggregation characteristics, foam molding properties and workability in foam molding processes.